Photometry - magnitudes

[Les Brand]

I'm using the Photometry process for observations of a variable (done with Johnson V filter) with Local Star DB V50 (I selected in Alignment tab) and finding that the Check star and 3 star magnitudes appear to be closer to the Johnson B mag shown on the image rather than the V magnitudes which is resulting in the calculated target mag fainter than the reported range on the annotated image. I thought the measured flux of the check star is normalised to V50/Gaia measurements and from that the instrumental mag calculated and target star mag calculated. In my case the magnitudes look quite wrong and I'm not sure why so advice most welcome.

[han.k]

Hi Les Brand,

Local database should be fine for Johnson-V. No problem to investigate. Can you share one orginal image and tell me which stars magnitude are reported with an offset?

Cheers, Han

[han.k]

I had some more time for looking to your screenshot. The VSP stars ar reported as follows:

000-BBB-588, AAVSO magn value 8.892 (0.02) , measured 9.0. So an offset of 0.008 magnitudes
000-BBB-598, , AAVSO magn value 10.967 (0.016) , measured 11.1. So an offset of 0.1032 magnitudes
000-BBB-613, , AAVSO magn value 11.618 (0.075) , measured 11.6. So an offset of -0.018 magnitudes

So two accurate matches. One outlier 000-BBB-598. Is the image calibrated with darks?

Han

[han.k]

A possible explanation is that BBB-598 is very red. The B-V is 1.450

See:
https://apps.aavso.org/vsp/photometry/?ra=00%3A23%3A43.00&dec=29%3A24%3A00.0&scale=D&orientation=visual&type=chart&fov=120.0&maglimit=14.5&resolution=150&north=down&east=right&lines=True

AUID    RA  Dec     Label   V   B-V     Comments

000-BJR-908     00:25:18.25 [6.3260417°]    30:05:40.7 [30.09463882°]   84  8.371 (0.034)28     0.913 (0.059)   
000-BBB-588     00:24:01.01 [6.00420809°]   29:28:22.2 [29.47283363°]   89  8.892 (0.020)10     0.580 (0.041)   
000-BBB-549     00:23:06.12 [5.77549982°]   29:12:05.7 [29.20158386°]   95  9.547 (0.060)16     1.098 (0.112)   
000-BBB-621     00:24:34.83 [6.14512491°]   29:22:14.3 [29.3706398°]    101     10.121 (0.073)16    0.563 (0.122)   
000-BBB-532     00:22:36.67 [5.65279198°]   29:14:52.1 [29.24780464°]   108     10.754 (0.068)16    1.251 (0.211)   
000-BBB-598     00:24:08.37 [6.03487492°]   29:26:47.1 [29.44641685°]   110     10.967 (0.016)10    1.450 (0.032)   
000-BBB-613     00:24:24.22 [6.10091686°]   29:26:32.3 [29.44230461°]   116     11.618 (0.075)16    0.490 (0.206)   
000-BMC-433     00:26:56.80 [6.73666668°]   28:52:04.0 [28.86777687°]   138     13.793 (0.021)29    0.604 (0.056)   
000-BMC-434     00:26:31.71 [6.6321249°]    28:43:30.9 [28.72525024°]   143     14.296 (0.055)29    0.675 (0.117)